Butyl polymer or rubber is well known in the art, particularly in its application in the production of tires.
The terms butyl polymer and butyl rubber are well known, interchangeably used terms of art and, as will be described in more detail hereinbelow, relate to a copolymer of an isoolefin and a conjugated diene. Generally, commercial butyl polymer is prepared in a low temperature cationic polymerization process using Lewis acid-type catalysts, of which a typical example is aluminum trichloride. The process used most extensively employs methyl chloride as the diluent for the reaction mixture and the polymerization is conducted at temperatures on the order of less than −90° C., resulting in production of a polymer in a slurry of the diluent. Alternatively, it is possible to produce the polymer in a diluent which acts as a solvent for the polymer (e.g., hydrocarbons such as pentane, hexane, heptane and the like). The product polymer may be recovered using conventional techniques in the recovery of rubbery polymers.
Elastomers go through a multitude of operations in the process of making a rubbery article. They are exposed to different shear rates and stresses during operations such as storage, mixing, milling, calendering, extrusion, injection molding and forming. The rheological behavior of the elastomers in the raw or compounded form is critical from the viewpoint of processability. These rheological properties are ultimately determined by the structural characteristics of the elastomer. For a review of processability issues and their relationship to rheological behavior, as well as structural characteristics of the polymer see, for example, J. L. White and N. Tokita: J. Applied Polymer Science, vol. 11, pp. 321-334 (1967) or J. White: Rubber Chem. Technol, vol. 50, pp. 163-185 (1976).
The requirements at different stages of processing are frequently contradictory. For example, it is desired that the polymer have certain strength in order to resist cold flow during storage or transportation. Higher elasticity or green strength can also be beneficial in forming operations to prevent excessive flow of the compound as it is shaped or formed. In this respect a high viscosity material is preferred, showing a high degree of elastic memory. It is generally believed that resistance to cold flow can be improved by increasing the molecular weight of the polymer or by increasing long chain branching. In contrast, during extrusion or injection molding it is often desirable to have a polymer with low viscosity and reduced elasticity in order to ensure high extrusion rates and dimensional stability. Rapid relaxation of stresses during these operations is also desirable so that the extruded article does not change its shape during the post-extrusion handling of the material. Increasing molecular weight or long chain branching can have a negative impact on these operations because of the increased elasticity. Very high elastic memory can also result in poor filler incorporation in a mixer or on a mill.
In addition to molecular weight and long-chain branching, the molecular weight distribution (MWD) of the elastomer is equally important. For example, narrow molecular weight distribution is believed to result in crumbling of the elastomer on a mill or in a mixer. Broadening of MWD can help to eliminate this problem. However, as MWD is increased, elasticity of the polymer will increase, resulting in an increase of die swell or compound shrinkage.
Dynamic testing is often used to assess rheological and processability characteristics of polymers. The key values derived from dynamic testing are the Storage Modulus (G′), Loss Modulus (G″) and Tangent Delta (tan δ). The Storage Modulus is a measure of stored energy or elasticity. The Loss Modulus is a measure of lost energy or viscous property. Tangent delta is the ratio of the two moduli (tan δ=G″/G′). Higher tan δ indicates that the sample will flow under stress rather than store the energy it was exposed to. Conversely, lower tan δ indicates that the sample will resist flow and show increased elasticity. Plotting the logarithm of tan δ as a function of the angular frequency (ω) provides very valuable information about the rheological behavior of the polymer. The slope of this curve can also be related to the molecular weight distribution and long chain branching of the polymer. Generally, a decrease in this slope is believed to indicate an increase in long chain branching or broadening of the molecular weight distribution. As the degree of branching is increased the slope decreases and eventually becomes zero. It is known that polymers close to, or at, the gel point have a frequency-independent tangent delta. For further information (see for example H. H. Winter: “Gel Point” in Encyclopedia of Polymer Science and Engineering, Supplement Volume, John Wiley & Sons, Inc. pp. 343-351 (1989), H. C. Booji: Kautschuk and Gummi Kunststoffe, Vol. 44, No. 2, pp. 128-130 (1991)).
The prior art contains numerous examples oriented toward the improvement of the processability of elastomers. As mentioned above, increasing long chain branching can reduce cold flow. One method of increasing long chain, branching is the introduction of a multifunctional monomer, such as divinyl benzene (DVB), into the polymerization mixture. When DVB is added to a polymerization mixture it will cause branching of the linear chains, as well as causing broadening of the molecular weight distribution. Using very low concentration of DVB will produce mostly linear chains containing only a few pendant vinyl aromatic groups. However, some of the growing chains will react with these pendant groups and the chain will grow through, resulting in an X-shaped molecule. This will double the molecular weight of the resulting polymer, leading to the broadening of the molecular weight distribution as linear and X-shaped molecules co-exist. As DVB concentration is increased, more chains will participate in this branching reaction and an increasing number of them will be able to react with more than one pendant group. This process will result in several jumps in molecular weight and each jump will lead to the formation of a new “population”. However, due to the statistical nature of the polymerization reaction, the final product will not be uniform, but will be a mixture of these different “populations”. There will be still linear chains present in the final product along with the X-shaped and other “populations” representing higher degrees of branching. Further increase in the amount of DVB will result in the formation of gel. The gel content will be dependent on the amount of DVB added to the polymerization mixture.
U.S. Pat. No. 2,781,334 [Welch et al. (Welch #1)] teaches the use of divinyl benzene in a butyl polymer production process to improve the green strength of the resulting polymer. Specifically, Welch #1 teaches that adding a small amount of DVB (0.1 to 0.8 weight %, preferably 0.4 to 0.8 weight %) to the polymerization system yields an oil-soluble, low gel, interpolymer. The physical properties are purportedly improved by a reduction in the cold flow of the polymer. However, a decrease in extrusion rate and an increase in die swell were also observed. This can be attributed to the increase of the molecular weight and long chain branching caused by the incorporation of DVB.
U.S. Pat. No. 2,729,626 [Welch et al. (Welch #2)] teaches that the use of 0.8 to 4 weight % DVB in the monomer feed produces a substantially insoluble terpolymer. This terpolymer can purportedly be used to make vulcanized products having improved physical properties with regard to modulus values. It is also claimed that copolymers containing no more than about 4% DVB have an extrusion rate sufficiently high to make extrusion practical.
U.S. Pat. No. 2,671,774 [McCracken et al. (McCracken)] teaches the production of products made using 4 to 10 weight % DVB in the monomer feed. Such products contain more than 80% gel. McCracken teaches that these products have greatly reduced cold flow. The achievable extrusion rate is purportedly higher than that of the unmodified polymer and die swell is decreased. McCracken also teaches that the blends of the product terpolymers with isoolefin-mutiolefin copolymers are also quite useful. However, the presence of gel in the polymer also resulted in the deterioration of cured properties (see Table II of McCracken). For example, tensile strength and elongation of the cured rubber decreased. This is not surprising, since the partially cross-linked rubber would not be able to homogeneously mix with the curatives and filler. In general, the presence of gel, especially in high amounts, in an interpolymer such as butyl polymer is not desirable because it makes the even dispersion of fillers and curatives normally used during vulcanization difficult. This increases the likelihood of under- and over-cured areas within the rubbery article, rendering its physical properties inferior and unpredictable.
These examples show that reduction of cold flow can be achieved by increasing long chain branching via the use of a multifunctional monomer. However, it has a negative impact on other aspects of processability, and formation of gel during polymerization is possible. High gel content results in inferior product properties. In some instances the use of a multifunctional polymer is cited as a method which is not preferred. For example, British patent 1,143,690 teaches that reduction of cold flow by the use of chemical crosslinking agents having polyfunctional groups inevitably results in the deterioration of the performance of the rubbery product and, sometimes, a considerable reduction in their processability. This is supported by a comparative example where DVB was used. The product obtained with DVB showed improved cold flow but its mill processability decreased significantly.
U.S. Pat. No. 5,071,913 [Powers et al. (Powers)] teaches that a good balance of processing characteristics (low cold flow and high extrusion rate) can be achieved by the addition of an effective amount of a functional reagent to the polymerization mixture. The functional reagent is selected from the group consisting of polymers and copolymers comprising functional groups capable of copolymerizing or forming a chemical bond with the product polymer—see column 16, line 24 to column 17, line 19 of Powers. Powers mentions the prior art related to DVB-modified butyl rubber and characterizes the prior art as deficient since it relates to polymers having a high gel content in the polymer product. Powers particularly prefers, as the functional reagent, such cationically active agents which do not contain active branching (crosslinking) functionality—i.e., the growing butyl chain can not propagate further if attached to the reactive site of the additive—column 15, lines 14-21. Indeed, Powers does not teach or suggest the use of a crosslinking agent during production of the butyl polymer and gives preference in the Examples to polymeric modifiers which tend to terminate the chain after attachment.
Despite the advances made in the art, there is an ongoing need for a butyl rubber which has an improved balance of (lower) cold flow, (higher) green strength, (faster) filler incorporation, (higher degree) of filler dispersion, (higher) stress relaxation rate and (lower) melt viscosity at high shear rates.
It is an object of the present invention to obviate or mitigate at least one of the above-mentioned disadvantages of the prior art.
It is another object of the present invention to provide a novel butyl polymer.
It is an additional object of the present invention to provide a novel process for producing a butyl polymer.
It is yet another objective of the present invention to provide a method for the prevention of gel formation when multifunctional crosslinking agents are used in the polymerization.
It is a further object of the present invention to provide a method for the purposeful alteration of the rheological properties of butyl polymer in order to achieve optimum performance in a given set of processing equipment.